This invention relates to photodetectors and more particularly to an arrangement of diodes placed at the output of a photodetector to convert the photo current to a log voltage without the utilization of resistors, transimpedance amplifiers and log amplifiers.
In one application it is very important for the output of a photodetector to have a very wide dynamic range, while at the same time responding to laser pulses in a ten nanosecond region. As is common in the detection of underwater mines, a laser illuminator is pointed down towards the surface of the ocean or other body of water and the returns from subsurface objects such as mines are then detected to determine the presence of such objects. Typically a mine is on the order of a few feet in diameter and it is only with difficulty that these subsurface mines can be detected at all.
In the past it has been the practice to provide many channels of information to a processor in a video receiver in order to be able to interpret the laser returns from the reflection of subsurface objects. Since the objects in question are only a few feet in diameter, laser pulses of equivalently short length are utilized. A short laser pulse is utilized to limit the amount of water excited to reduce light diffusion and to be able to detect these small targets.
Not only are laser pulses of necessity relatively short, it will be appreciated that one would expect strong returns from a mine which is only a few feet below the surface of the water, and extremely weak returns from mines at greater depths.
The problem of detecting mines at unknown depths is that while a relatively large signal is returned from shallow mines, the signals from deep mines are weak due to the attenuation of the laser pulse as it passes through the water.
What is therefore required is a detector which has a relatively wide bandwidth to be able to detect short pulses, while at the same time having a dynamic range which covers returns not only from shallow mines, but also from the deep ones as well.
In the past, the video receivers utilized in this application incorporate a photodetector coupled to a transimpedance amplifier to convert the output current of the photodetector to a voltage. Thereafter, the output of the transimpedance amplifier is coupled to a log amplifier which has an output which is the log of the input voltage. Thus the log amplifier provides dynamic range extension by compressing the input signal range for subsequent analog-to-digital conversion.
The problem with such a configuration is that while it does in fact achieve a log output, the log amplifier has a low bandwidth, which in the best of circumstances is no more than a 100 MHz. Additionally, log amplifiers dissipate large amounts of power, and have a somewhat limited dynamic range. Moreover, there is a high noise floor associated with such an arrangement, making the detection of weak signals from deep mines difficult.
Conventionally, photodetectors are provided with a shunt resister to ground to convert the photo current to voltage. While the advantage of such a system is that it is passive and is small in size, this arrangement has a low bandwidth. Moreover, the resistor termination method results in a high output impedance and an exceptionally high noise floor. Additionally, there is of course no logarithmic relationship of the output voltage to the input current, which is not useful in the above noted in the application.
As mentioned hereinbefore, it is possible to convert photo current to a voltage with an inverting or transimpedance amplifier which has the advantage of a low output impedance, but the disadvantage of a low bandwidth, coupled with high power dissipation and a high noise floor. As a result, prior approaches to the provision in a suitable video receiver have been inadequate.
In order to provide adequate dynamic range as well as logarithmically related output signals for the detection of very short LIDAR pulses, in the subject system the photodiode output is provided with a series of junction diodes connected between the photodiode output and ground. The result of such a termination scheme is that the output of the photodetector is immediately converted to a log voltage. The reason is that the junction diode acts as a nonlinear resistor which compresses the signal. The non-linearity of the resistance provided by the diode is a log function in which the input current is compressed by this nonlinear resistive function. Thus the output voltage is the log of the input current.
The resulting detector has a wide bandwidth utilizing passive, small size components. Moreover, the subject system provides a low output impedance with a full dynamic range and low noise. The dynamic range of the subject system may be increased by increasing the number of series-connected diodes between the output of the photodiode and the ground.
In one embodiment a PIN photodiode is utilized, which produces reversed current in response to incident light. The PIN diode may be reverse biased to improve its quantum efficiency. The amount of current produced as a function of incident light is in terms of amps/watt. The photodiode acts as nearly an ideal current source especially when substantial reverse biases are applied to the diode. When the output of the photodiode is terminated by one or more series connected junction diodes, then the classical junction diode equation which relates the junction diode terminal voltage to its current is one of a logarithmic nature. Thus, the utilization of a junction diode provides a logarithmically related output voltage.
Since the temperature dependence of the diode amplitude response is exactly that of a junction diode, a reference junction diode can be used to compensate the output voltage so as to normalize against temperature variation.
Further, the subject diode termination scheme allows the PIN photodiode to dominate the noise floor and thus provide a low additive noise factor in the conversion of photodiode current to log voltage.
It is noted that noise in both the photodiode and the termination diode is composed of two components namely the Thermal Johnson Noise and the Generation/Recombination, GR, Noise. The GR noise is proportional to the rate and number of electron/hole pair generations. Recombinations which will always be greater in the PIN diode due to the presence of the large intrinsic region. Thermal noise is proportional to the effective junction area and temperature. For a given temperature, the PIN diode will exhibit greater thermal noise due to its intrinsic region. Therefore, the termination diode noise will be less than that of the photodetector for any practical number of termination diodes.
Additionally, the inherent photodetector bandwidth is on the order of a hundred megahertz. This is limited by the minority carrier lifetime of the PIN diode as well as its output capacity, generally a few picofarads.
As mentioned hereinbefore, other conventional signal conversion methods severely limit the inherent detector bandwidth. Schemes involving active amplifiers or log amplifiers push the overall bandwidth below 100 megahertz. Additionally for large signals, these amplifiers impose slew rate limitations that are even more restrictive. The diode termination method described hereinabove has no slew rate limitation.
Also, additional series-connected termination diodes increase the output voltage for a given incident light level by N, where N is the number of diodes. The desired signal level is additive and the diode junction noise increases as the square root of N. Therefore, the signal/noise ratio for the diode termination string improves in proportion to N.
It will also be appreciated that the overall termination string capacitance decreases in proportion to 1/N, this acts to improve the bandwidth as more diodes are added.
What is accomplished by the termination of the output of a photodetector with junction diodes is to provide dynamic range compression, while preserving a wide bandwidth. This is accomplished without the cost and space burden of active amplifiers and the accompanying high power dissipation. Additionally, the subject system eliminates the problem of high noise floors associated with the above approaches.